1. Field of The Invention
The invention relates to a process for regulating the breaking length ratio of a manufactured paper web by altering the microtuibulence in the vicinity of the headbox nozzle. The invention further relates to a paper machine with a headbox suitable for performing the process according to the invention.
2. Discussion of Background Information
Together with other quality-related parameters, the breaking length ratio of a paper web represents an important criterion in defining the properties and quality of manufactured paper. The breaking length ratio L/Q is defined as the ratio of the breaking length L in the longitudinal direction to the breaking length Q in the transverse direction of a paper web. The breaking length provides a measure of the tensile strength of the paper. The breaking length specifies how long a strip of paper can be before it tears under the load of its own weight. The breaking length ratio L/Q is obtained as a dimensionless number by measuring the breaking length in both the longitudinal and transverse directions and then taking the ratio of the two.
Different paper requirements place different demands on the breaking length ratio L/Q. This is especially true for papers processed m high-speed printing presses, for example, which require that the breaking length in the longitudinal direction L, be correspondingly higher, in order to ensure trouble-free operation of the printing press. In such a case, a high breaking length ratio, which also corresponds to a high Concora value, is required.
It is noted here, that according to the used finish and the production process on the paper machine, the total breaking length potential is limited. This fact is given more or less by the geometric mean (GM) value of the breaking length GM=(L/Q).sup.1/2. Therefore, increasing L/Q means increasing L as well as reducing Q at the same time. Reducing L/Q means reducing L and increasing Q.
Some papers require a high uniform tensile strength in both the longitudinal and transverse directions. For example, these may include format papers and many packaging papers. In this case, the breaking length ratio L/Q should be as close to 1 as possible, i.e., L and Q should each approximate the same value. This means that the tensile strength of the paper is essentially uniform in all directions.
Other papers have different requirements. A further example is provided by the fact that there is no uniform evaluation of the center layer of corrugated board, referred to as flute or fluting. Some customers evaluate flute with the Concora Medium Test (CMT), for which a relatively high breaking length ratio is needed. At the same time, the Short Compression Test transverse to the web (SCT.sub.TRANS), for which the lowest possible breaking length ratio L/Q is needed (reducing the CMT value), is also commonly used for evaluation, This means that the manufacturers of flute are forced to alternately produce flute with high and low breaking length ratios when supplying to different customers.
The prior art discloses a process for influencing the breaking length cross profile of a moving fiber web. Such a process is described in European patent application EP 0 774 540 A2. The object of this invention is to bring uniformity to the cross profile of the breaking lengths of a paper web, which profile varies across the web width and has values at the edges in particular that deviate from the average.
However, this reference does not specify a process for adjusting and regulating of a paper machine in a manner disclosed by the instant invention. It merely suggests the possibility of exercising influence on the cross direction breaking length profile of the paper web in the edge zones.
Thus, the prior art does not address or solve the problem of how a specific breaking length ratio L/Q, corresponding to customer requirements, can be controlled in the manufacturing of paper, and especially doing so in an ongoing manufacturing operation.